Uranium-lead isotopic age of monazites of plagiogneyses of the Ingulo-Inguletse series (Ingul megablock of the Ukrainian Shield)

Abstract

Among the supercrustal formations of the Ingul megablock, Paleoproterozoic rocks, represented by the Ingul-Ingul series, are significantly predominant. In general, the composition of the Ingul-Ingulets series includes (from bottom to top): Zelenorichen, Artemiv, Rodioniv, Kamyano-Kostovatsky (age analogue of Spasivsk) and Roschakhiv (age analogue of Chechelivsk) worlds. The Chechelivsk suite is represented by biotite and garnet-biotite gneisses, places with sillimanite and cordierite, as well as underlying layers of pyroxene gneisses and schists. Suite rocks are widespread within the Ingul Synclinorium, in the framework of the Korsun-Novomyrhorod pluton and in the northern part of the Bratsky Synclinorium. Currently, the age of monazites from plagiogneisses distributed in the Novgorodkiv quarry and south of the village Lavrovka has been determined using the uranium-lead isotope method. The obtained monazite isotopic age values of 2022.9 ± 6.7 Ma and 2031.5 ± 1.5 Ma, respectively, can only serve as the upper age limit for the formation of the Ingul-Ingul series. Analysis of the internal structure of zircon crystals from plagiogneiss distributed south of Lavrivka, showed that the majority of zircons are complex. In almost all crystals there are mainly thin shells of light pink zircon. Light pink zircon cores are found in the middle of the light pink crystals. Crystals with a brown color have a more complex structure. Along with isometric cores of light pink zircon, cores with fine «magmatic» zoning occur, as well as highly fractured uniform brown cores and complex cores.On this basis, it is assumed that this plagiogneiss was formed by clastogenic material from several sources.